In recent years, regarding laser printers and copying machines utilizing electrophotography, a process speed has been increased at a fast pace, and a toner excellent in development performance, transferability, and low-temperature fixability has been demanded. In particular, since the low-temperature fixability contributes to saving of power consumption, it is regarded as an essential factor in recent development of toners of the type that is strongly required to be adapted for environmental countermeasures.
Meanwhile, with increasing market expansion of laser printers and copying machines, toners have been demanded to stably exhibit the performance even in use in environments ranging from low temperature and low humidity environments to high temperature and high humidity environments. Therefore, researches are directed to development of a toner having excellent durability in an environment different from the low-temperature fixability.
For example, Patent Literature 1 discloses a toner including toner particles containing at least a binder resin, a release agent, a polar resin and a sulfur atom-containing polymer and inorganic fine particles, wherein the polar resin is a vinyl polymer having at least one of a carboxyl group and a hydroxyl group. In Patent Literature 1, when the interfacial tension to water of the sulfur atom-containing polymer dissolved in styrene which is measured by the pendant drop method is a (mN/m) and the interfacial tension to water of the polar resin dissolved in styrene which is measured by the pendant drop method is b (mN/m), a and b are specified as follows:a+5.0≤b  Formula I17.0≤b≤24.0  Formula II
An object of the invention of Patent Literature 1 is to provide a method in which when the material that satisfies the above Formula I is selected, the sulfur atom-containing polymer tends to be preferentially located near the surface of toner particles, then even if many sheets of paper are printed, the occurrence of defects such as developing stripes, dropping, and fogging is prevented and further the image density becomes stable. In the invention, another object is to provide a method in which when the material is selected so as to satisfy the above Formula II, the moisture content on the surface of toner particles are stably maintained in environments ranging from low humidity environment to high humidity environment.
Patent Literatures 2 to 3 disclose toners in which the interfacial tension to water when yellow pigment and magenta pigments as colorants are dispersed or dissolved in styrene satisfies the specified relationship, in addition to the relationship in Patent Literature 1.
However, in the case of the toners obtained by the methods described in the Patent Literatures, it is difficult to achieve a balance between decrease in fixing temperature and shelf stability at high temperatures and satisfy demands of continuous printing durability and printing durability after storage at high temperatures. Further, the print quality in a different environment, particularly a high temperature and high humidity (H/H) environment have been sometimes insufficient.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2010-91704
Patent Literature 2: JP-A No. 2011-150125
Patent Literature 3: JP-A No. 2011-215179
In the methods disclosed in Patent Literatures 1 to 3, the interfacial tension to water of styrene and the interfacial tension to water of a styrene dispersion of a toner material are used in the selection of the toner material. However, Patent Literatures 1 to 3 disclose polymerizable monomers, for example in addition to styrene, other styrene monomers such as methylstyrene, (meth)acrylic acid ester monomers such as methyl methacrylate, and ene-based monomers such as cyclohexene. Particularly, if a monomer having a high polarity, like (meth)acrylic acid ester monomers, is used with styrene, the interfacial tension to water is relatively low as compared to the case of using only styrene. In an aqueous medium, styrene tends to agglutinate inside oil droplets as compared to the monomer having a high polarity. Thus, the distribution state of the polymerizable monomer in the oil droplets is changed depending on the use of the monomer having a high polarity. Therefore, the above Formulae I and II using the interfacial tension of the styrene dispersion do not accurately describe the actual state of the oil droplets of the polymerizable monomer composition.